Enabling adapter literal definition in Db Validators

Changed the  Zend\Validator\Db\RecordExists and Zend\Validator\Db\NoRecordExists with the ones implemented in my PR in zf-content-validation.
Added a text input for setting the adapter in the validator definition
![capture decran 2014-02-27 a 16 16 44](https://f.cloud.github.com/assets/5320213/2283838/5579af6c-9fc2-11e3-9361-192b84fb82ef.png)